1. Field of the Invention
This invention relates to a method for making a composite aluminum article. This invention particularly relates to a method for making a composite aluminum article, wherein the surface of an aluminum material or an aluminum alloy material is imparted with excellent lubricating properties.
2. Description of the Related Art
As a method for imparting lubricating properties to oxide films of aluminum alloy materials which films are formed by anodization, methods described below have already been proposed.
Specifically, it has been proposed to impregnate unsealed pores or unsealed cracks in an oxide film formed by an anodizing process with a lubricating oil.
However, the aluminum alloy material obtained from such a method has drawbacks in that, for example, it cannot be used in applications where a solvent is used.
It has also been proposed to use solid lubricants for the purpose of imparting the lubricating properties. For example, Japanese Unexamined Patent Publication No. 56(1981)-130489 discloses a method wherein an aluminum alloy material is immersed in a solution containing a metal oxyacid salt (tetrathiomolybdate), and then treated by anodic secondary electrolysis, so as to fill pores in the oxide film formed by anodization with a metal oxide (molybdenum disulfide). A method has also been proposed wherein an aluminum alloy material is immersed alternately in a metallic soap solution and an acid. A method has additionally been proposed wherein an aluminum alloy material is immersed alternately in a liquid containing a metal sulfide and in an acid. Further, a method has been proposed wherein electrolytic pores are filled with a metal soap or a metal sulfide.
However, the proposed methods have drawbacks in that the formed film exhibits a high coefficient of wear (0.2 to 0.3), poor durability, and insufficient self-lubricating properties. Actually, the proposed methods have not been put into practice for the purposes of imparting lubricating properties to sliding parts which requires good lubrication characteristics. This is presumably because the degree of wear of the anodic oxide film in itself is high.
Also, like a method typified by the method disclosed in Japanese Patent Publication No. 52(1977)-39059, a method has been known wherein pores of a hard anodic oxide film are impregnated with a polytetrafluoroethylene (hereinafter referred to as the "PTFE"). This is a composite film forming method utilizing the characteristics of a PTFE resin which exhibits a low coefficient of friction and water- and oil-repellent properties. Specifically, in this method, a hard anodic oxide film having a thickness ranging from 20 .mu.m to 50 .mu.m is formed and is then impregnated with PTFE to a depth of approximately 10 .mu.m from the surface of the film.
It is said that the aforesaid method utilizing PTFE can result in excellent wear-resistant properties. However, it is doubtful that PTFE will penetrate into the pores of the hard anodic oxide film with certainty. Also, the adhesion of PTFE is not sufficient, and improvements are rather poor in the sliding characteristics of precision parts which requires high accuracy and of compressor parts which requires high air tightness. Additionally, sufficient resistances to abrasive wear and adhesive wear cannot be obtained. Thus there remain problems to be solved with regard to the use of this method for parts which have to have high overall durability.
As described above, the conventional method of treating a hard anodic oxide film for imparting the lubricating properties thereto exhibit only insufficient improvements in the sliding characteristics of precision parts which have to show high accuracy and of compressor parts which have to show high air tightness.